Invisible wrote:
> 
> Yeah, this is the thing. I had assumed that the digital devices you buy 
> in the shop are carefully designed to behave in a simple, predictable 
> mannar, even though their internal construction is a tangle on analogue 
> devices. I thought you could just treat a 7400 as a black box. 
> Apparently not...
> 

To a point you can - knowing that point is the trick - and likely some 
of what you are facing.

>> And as you have found out with LEDs, even when you think you are only 
>> dealing with digital, you are dealing with analog as well.
> 
> The LEDs actually worked just fine. It's the gate driving them that 
> didn't do what it was ment to.
> 

One think to keep in mind, you are best off not trying to drive the LED 
directly from the chip.  Some chips can do it, but most cannot.  Lots of 
times you need to put a *switch* that the gate switches.
see:
http://www.rason.org/Projects/transwit/transwit.htm
http://www.eecs.tufts.edu/~dsculley/tutorial/transistors/transistors1.html

remember, you need a resistor in the path from + to ground with the LED.


>> Even something as simple as a button or switch can cause problems.
>> When you close the contact there are literally thousands of 
>> connections made and broken in the very short time before the contact 
>> is completely closed.  If you have a counter, it might count 4 or 5 on 
>> each button press, not just one.
> 
> I wouldn't find this surprising. Rather, to be expected.
> 


Good, something won't catch you by surprise.  But it's all those little 
things that can add up and frustrate you.


>> That's what debounce circuits are for, but they can be analog - go 
>> figure.
> 
> Presumably a debounce circuit is merely a low-pass filter?
> 

can be - it can be implemented purely analog, or it can be filtered in 
programming.
You can also use some logic gate tricks to debounce.


>> Don't try to build a rocket ship - build something simple.
> 
> Heh. I got stuck just trying to get a truth table out of a logic gate. :-S
> 

So, keep trying to do it.  That's how you start.  Overcome the things 
that keep it from working.  How else do you learn.
You can certainly ask questions here - you've already gotten an ear full.


>> Get a book that doesn't just say hook this wire here and there, but 
>> that actually goes through how it all works.
>> The CMOS Cookbook I pointed out earlier is a good book in that reguard.
> 
> ....does it matter that I'm using TTL?

TTL & CMOS are very similar in their logic, but completely different in 
how they work internally.

You can't just hook the two together.  At you level you are better off 
sticking with one or the other.

CMOS is much lower power and is what is typically found in newer 
electronics - tho TTL has its place as well.

There is a TTL cookbook by the same author - it is basically the exact 
same book.

You can use either book, they are both good starts.


The TTL Cookbook is what opened my eyes that I could actually do 
electronics and it was relatively easy.  I had to return it as it was a 
loaned book, but then I bought the CMOS cookbook and took off from there.



I highly recommend either of the books if you are really interested in 
learning some about electronics.  Unless you have $100 in TTL chips, I 
recommend that you go the CMOS route as it is more likely what you would 
encounter when dealing with more specialized chips (think CPU).

If you get one, start reading it from the beginning and learn the 
fundamentals.

Post a reply to this message